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Electrically conductive polymer: how scientists made plastic from metal



Every year, more and more gadgets acquire new talents, including the ability to connect with each other via the Internet. The concept of “Internet of things”, which arose at the turn of the century, takes on a clearer outline, however, for the full utilization of this idea, additional technical innovations are needed that can solve a number of problems, including the charging of wearable electronics. One of the most popular and futuristic solutions is to use the heat of the human body. And for this we need light, non-toxic, wearable and flexible thermoelectric generators. Scientists from Nagoya University (Nagoya, Japan) have proposed the use of plastic. How the electrical conductivity of plastic is related to its structure, how to manipulate this parameter,and how effective is the use of plastic in creating thermoelectric generators? The answers to these questions await us in the report of scientists. Go.

Study basis


The human body can hardly be called a source of large amounts of heat. However, in the aspect of wearable electronics, the heat of our bodies can be used to support the operation of our gadgets. However, modern electrically conductive polymers cannot yet boast of their thermoelectric characteristics. To change this, you need to look inside the structure of the material and understand how everything is arranged there.

One of the key performance parameters in thermoelectric devices is the power factor: P = S 2 σ, where S is the Seebeck coefficient * , and σ is the electrical conductivity.
* ( ) — , .

— , .
If it is assumed that most conductive polymers, except poly (3,4-ethylenedioxythiophene), do not show P maxima upon doping of carriers. This means that P continuously increases with increasing σ for higher doping levels. The rationale for this non-standard behavior is the power relation * S ∝ σ −1 / s , where s is 3 or 4 (in most cases).

Power law * - in statistics, this is a functional relationship between two quantities, when a change in one leads to a proportional change in the second quantity.
A similar effect occurs due to the disorder of polymer films, where structural / energy disturbances within the film influence charge transfer.

Recent studies have shown that controlling the effects of randomness on the charge transfer process is an essential mechanism in achieving control over P by modifying the empirical S - σ ratio .

In this work, scientists show that the empirical S-σ ratio of a conductive polymer can indeed be changed by controlled doping of the carrier.

The protagonist of the experiments was the PBTTT polymer, or more extensively, poly [2,5-bis (3-alkylthiophen-2-yl) thieno (3,2-b) thiophene]. The choice of this particular polymer is not accidental, since it exhibits the highest conductivity (S / cm, i.e. Siemens per centimeter) among semi-crystalline polymers, which is achieved by doping with 4-ethylbenzenesulfonic acid (C 2 H 5 C 6 H 4 SO 3 H) .

Unlike conventional transistors using a solid state gate isolator, this technique allows you to continuously monitor the doping level of conductive polymers to very high concentrations through an electrochemical process. Thus, electrolytic-controlled PBTTTs allow one to fully consider the thermoelectric properties of PBTTTs, including the metallic state.

Research results


First of all, scientists studied the thermoelectric properties of thin PBTTT films doped with an electrolytic gate.


Image No. 1

Image 1A shows a diagram of the experimental setup, which allows one to simultaneously measure S and σ when the carrier is doped. Figure 1B shows a snapshot of the structure of a thin-film transistor (TFT from thin-film transistor ) of a PBTTT polymer. The concentration of charge carriers can be continuously monitored by applying a gate voltage ( V g ) throughout the entire process of electrochemical doping, where dopant ions penetrate the bulk film. σ is determined by the current-voltage characteristics obtained by applicationV g ( 1C ). S is determined by the slope of the thermoelectromotive force (∆ V ) depending on the temperature difference (∆ T ) between the electrodes ( S = ∆ V / ∆ T ) for each V g ( 1D ).

Figure 1E shows the dependence of σ on S (above) and σ on P (below), obtained for two independent devices at room temperature. Due to the constant doping of charge carriers, the data obtained from both devices had a rather small scatter. And the observed form of the relationship S- σ is reversible if V g does not exceed the threshold of degradation of the device.

The first thing that was noticed is that P shows a clear maximum above 100 S / cm. The appearance of a maximum of P is expected in the following two cases. In the first case, these are ordinary nondegenerate semiconductors, where the S - σ relation is described by the logarithmic relation S ∝ ln σ. However, the observed slope of the Yonker curve (the dependence of the Seebeck coefficient on the logarithm of the conductivity) demonstrates gradual changes of about 10 and 100 S / cm. And this suggests that a conventional thermally activated process cannot explain the obtained experimental data.

In the second case, it was noted that the maximum P is most often observed if the electronic state changes from non-degenerate to degenerate upon doping of the carrier. In this case, the S - σ (or P - σ ) ratio can be divided into two regions at the doping level giving the maximum value of P , which reflects a fundamental change in the electronic properties of the doped materials.

The S - σ ratio ( 1E ) follows the empirical S ∝ σ -1/4 (or P ∝ σ -1/2 ) in the low conductivity region ( σ <100 S / cm), but with an increase in σ, the S - σ ratio moves away from this values ​​approaching S ∝ σ -1 (dotted line on 1E ).

Since the electrochemical doping process involves the penetration of dopant ions into the film, it is also necessary to investigate the possibility of structural modification of the molecular arrangement during the doping process, which can affect the thermoelectric properties. To do this, scientists conducted experiments on x-ray diffraction (GIXD) on a doped polymer, the results of which are shown in the image below.


Image No. 2

In an untouched film ( V g= 0 V), distinct scattering peaks were observed outside the (h00) plane, corresponding to a plate structure up to the fourth order, and also a peak in the (010) plane, corresponding to a π-π stack, indicating the highly crystalline nature of the PBTTT thin film. Atomic force microscopy (AFM) of the film surface also confirmed high crystallinity. When the carrier is doped, the peak profiles show obvious changes (peak (100) at 2B and 2D ; peak (010) at 2C and 2E ).

The scattering vector q z of the peak (100) is continuously shifted to lower values, while | V g | increases due to the expansion of the distance between the plates from 23.3 Å at Vg = 0 V to 29.4 Å at V g = −1.6 V ( 2D ). This expansion is caused by intercalation * of bis (trifluoromethanesulfonyl) imide (TFSI) anions in the film
Intercalation * is the reversible incorporation of molecules, ions or atoms between molecules or groups (layers) of atoms of another type.
However, in comparison with previous studies, the increase in the lattice step in this case is much larger (~ 6), which is close to the length of the TFSI anion (~ 8.0). This result implies that the TFSI molecules are located in the interlamellar position (between the films) to form an end-to-end configuration with alkyl side chains ( 2F ).

Even with such a large lattice expansion, no expansion of the lines of diffraction peaks was observed, i.e. the crystallinity of the lamellar structure does not deteriorate due to anionic intercalation.

In addition, anionic intercalation occurs reversibly, as evidenced by the fact that the distance between the gratings is restored close to the original value when a positive voltage after doping is applied.

There was also a clear shift of the scattering vector q xy of the (010) peak towards higher values ​​during doping, which indicates a reduction in the π-π stacking distance ( 2C and 2E ).

In total, the data of the above experiments clearly indicate that the system does not exhibit structural degradation upon doping.

Next, EPR (electron paramagnetic resonance) spectroscopy was performed.


Image №3

At 3A shows a diagram of TFT (thin film transistor) liquid with an ion-gate that allows simultaneous measurement of ESR and conductivity when applying V g .


Addition to 3C

In the case of using negative V g , a clear EPR signal of positive carriers (polarons) is observed in the PBTTT ( 3B ) chain . A signal is observed with a g value of about 2.003, regardless of the V g value , if an additional external magnetic field ( H ) is used, which is perpendicular to the substrate. This result indicates that the carriers are in the region with edge-on (i.e., edge) orientation ( 3C ), which is consistent with the GIXD results showing the absence of crystalline fracture in the doped film.

From the integral intensity of the EPR signal, it was also possible to determine the spin susceptibility (χ) of the doped film. On 3Dshows a graph of χ versus σ , obtained simultaneously with the EPR measurements. In the region of low conductivity, with an increase in σ , a sharp increase in χ was observed.

In lightly doped regions where polarons are magnetically isolated, the spin susceptibility follows the Curie law:

χ = Ng 2 µ B 2 S (S + 1) / 3k B T , where N is the total number of spins.

In this case, the spin susceptibility is proportional to the carrier concentration n ; therefore, the relation χ ∝ σ must be in a state of constant mobility. This ratio is indeed observed in the region of very low conductivityσ <0.01 S / cm ( 3D ), which indicates the dominance of isolated polarons in charge transfer.

If the value of σ exceeded 1 S / cm, then a clear broadening of the lines ( 3E ) was observed , indicating a completely different dynamics of spins in this region. In such a situation, carrier delocalization was observed. This indicates that the energy disorder does not dominate in the transfer process in the crystalline domain with σ above 1 S / cm.

When delocalized carriers after doping form a degenerate (or metallic) state, the Curie law *is no longer satisfied, and Pauli’s spin susceptibility takes its place when χ is proportional to the density of states at the Fermi energy level, and not to the carrier concentration n .
Curie law * - the degree of magnetization of paramagnets is inversely proportional to temperature in the case of a change in temperature and with a constant external field.
An almost complete saturation of the increase in χ was also observed in the case of an increase in σ above 1 S / cm, which includes line broadening due to carrier delocalization. This confirms the formation of a degenerate (or metallic) electronic state in domains with edge orientation.

In the case of σ ~ 100 S / cm EPR signals not showed no abnormality and the ratio S - σ demonstrated deviation from S - σ -1/4 ( 1E ). This suggests that the thermoelectric properties change independently of the microscopic electronic state in the domains.


Image No. 4

Figure 4Ashows the temperature dependence of σ obtained at various values ​​of V g . At room temperature ( σ RT ) and increase | V g | an increase in conductivity was observed. With a sufficiently high value | V g | a region of negative temperatures ( dσ / dT <0) appears , indicating a metallic state.

The metallic state was observed even at temperatures below 200 K and | V g | > 1.7 V, which is lower than the freezing temperature of the electrolyte. These observations were additionally confirmed by measuring the magnetoresistance at V g= -2.2 V and 150 K ( 4B ).


Image No. 5

In conclusion of their work, the researchers analyzed the relationship between charge transfer and thermoelectric properties. Figure 5A shows S - σ ratios at room temperature obtained both in this study and in other studies using other doping methods.

Scientists point out that the conductivity at which the S - σ ratio deviates from an empirical value agrees quite well with the conductivity at which charge transfer is observed in metals, i.e. σ RT ~ 100 S / cm.

This confirms that the ratioS ∝ σ -1 observed in the high-conductivity region does indeed follow the Mott equation, reflecting the metallic nature of the system. In contrast, σ exhibits a nonmetallic temperature dependence in the region of σ RT <100 S / cm, although the microscopic electronic state in the crystalline domain is metallic above 1 S / cm. This result indicates that the macroscopic charge transfer process is mainly limited by structural heterogeneity, such as domain walls, and not by charge capture inside crystallites.

Researchers recall that the macroscopic charge transfer process in polycrystalline polymer films is modeled by binding molecules between crystalline domains ( 5B) In this case, the local structure of the binding molecules significantly affects the charge transfer process.

In this case, the domain combination should be sufficiently sensitive to doping conditions, probably due to the structural / energy disorder of isolated dopant-induced binding molecules. In other words, moderate doping using the existing technique for gating electrolytes makes it possible to efficiently connect crystalline domains, which leads to a macroscopic metal transition giving the maximum power factor in a PBTTT thin film.

For a more detailed acquaintance with the nuances of the study, I recommend that you look into the report of scientists and additional materials to it.

Epilogue


An electrically conductive polymer is not something new, however, in this work, scientists were able to improve it, thereby increasing its thermoelectric characteristics. The bottom line is that thin films inside the polymer consist of crystalline and non-crystalline parts, which greatly complicates the process of studying the properties of polymers, let alone manipulate them.

However, in this study, the PBTTT polymer was used, in which a thin layer of ionic electrolyte gel was added to increase conductivity. To successfully connect these two elements, it was necessary to apply a certain voltage, which also allowed us to evaluate the structural properties of the resulting system.

The resulting polymer in its conductive indicators was more like metal than plastic. However, this was achievable only under certain conditions (voltage and temperature). In the future, scientists intend to continue their work, concentrating on improving the methodology for converting polymers by possibly changing the methodology for the formation of the system (search for an alternative to alloying).

Thank you for your attention, remain curious and have a good working week, guys. :)

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